In the chemical field, in particular in natural gas and oil transportation, water containing aggressive components (sulfur, salt) occurs in considerable quantities. This water is transported through welded steel pipes at times under very high pressures (up to 200 bar). If these are not isolated from the inside, they are destroyed by corrosion in a short time, which adversely affects the operation and considerably endangers the environment.
To reduce corrosion it has been suggested to install plastic tubes within the steel pipes. This, however, leads to an undesirable reduction of the pipe cross section.
For the prevention of this effect, it is known to coat the interior of pipes with material which is resistant to the accumulated aggressive media. This coating is carried out in pipes of large diameters by means of mobile vehicles which spray coating material.
This is not possible for pipes of small diameters. Instead, they may be coated, for example, by a package consisting of two pigs between which the in general fluid coating material is located, which is drawn back and forth through the pipe. Such a procedure has the disadvantage that the weak points which occur in the region of uneven parts of the internal pipe surface, particularly at the weld seams, where the coating material is stripped off by the second pig and thus form attack points for the aggressive media.
Another process for coating is airless paint spraying (the so-called airless-process). In this, the coating material from a tank located outside the pipe is pumped to a nozzle in the pipe by means of a high pressure pump and a high pressure tube. The high pressure in the tube causes the material to be sprayed and coat the internal surface of the pipe. The disadvantage of such a system is that, as a result of the considerable pressure loss caused by friction in the high pressure pipe only limited pipe lengths may be coated.
Even in oil and natural gas transportation, piplines are provided for the transport of required substances which may in parts extend underground for long stretches and so the new coatings of the inner tube surfaces required at regular yearly or half-yearly intervals causes a considerable expenditure. In order to effect the interval coating by one of the known apparatuses, stretches of about 300 m are dug up and the tube cut apart.